The present invention relates to a method of controlling and/or measuring the thickness of layers, such as the thickness of surface layers, and more particularly, although not exclusively, to controlling the thickness of seizing glue on paperboard, the thickness of kaolin coatings on paper and/or paperboard, the thickness of glue coatings when glueing or bonding plastic foil, the thickness of varnish layers on wood, the thickness of priming coatings of clear varnish on light-coloured woods, and the thickness of plastic coatings on plastic mats or floorings.
Methods for, e.g., controlling and monitoring the application of glue and glue coatings are at present not usually applied in industrial manufacturing processes.